Harm's target is "creature touched". If you want to touch someone who doesn't want to be touched, you must make a touch attack. Most enemies don't want to be afflicted by a Harm spell. You must make a touch attack to Harm. You may also hold the spell indefinately if you do not discharge it on the round Harm is cast. You can use reach spell on Harm as it has a range of "touch" which is referenced by reach spell.
Ringtail is right.
I just wanted to chime in that on the messageboard I had the text really small and I thought this was a thread on Ham and Touch attacks. And I wanted to know more.
Most touch attacks state that it requires a melee touch attack to hit.
Most touch attacks do not have save for Half.
Harm has a safe for half and does NOT state that it requires a melee touch attack in the description (though the target is person touched).
After casting Harm do you then have to make a touch attack? I think not, but also think it could thus not be held. Would using the metamagic Reach effect this?
Many touch attacks have a save for half(or more).
Any/All Inflict Wounds spells
Ray of Enfeeblement
Slay Living
Disintegrate
a few off the top of my head.